Separation, purification, and analysis of metal ions is of great importance in many diverse areas. Examples are hydrometallurgy and electroplating processes, metal recovery and waste treatment, the preparation of high purity materials for the electronic and laser industry and the analysis of trace metals in body fluids. Two of the methods that are inherently applicable to most of these fields are solvent extraction and membrane technologies. Solvent extraction has extensively been applied for the separation of metal ions in bulk quantities in hydrometallurgic processes. Over the years, much emphasis has been placed on improving the performance of a very small number of simple ligands by carefully modifying extraction conditions such as pH, masking agents, stripping agents and the nature of the organic solvent. On the other hand, only marginal efforts have been devoted to the design of tailor-made ligands that would be inherently fit to extract a specific metal ion in the presence of many others. The subject of the present application involves binders that selectively extract a specific metal ion from a mixture of many. Examples are a hexavalent ligand that separates Fe.sup.3+ from a mixture of Fe.sup.3+ and Cu.sup.2+ and a hexavalent ligand that preferentially extracts Zn.sup.2+ from a mixture of Zn.sup.2+ and Cd.sup.2+. The iron binders among these compounds act as growth promoters of microorganisms by simulating the properties of the natural iron binder ferrichrome.